In the exploration of oil, gas, and geothermal energy, drilling operations are used to create boreholes, or wells, in the earth. In many locations, it has been found to be advantageous to be able to track the position and direction of the subterranean drill bit during the drilling process. Measurement-while-drilling (MWD) tools have been developed for this purpose.
MWD tools typically have electromechanical accelerometers mounted with their sensitive axis aligned orthogonally to the spin axis and to each other. Micro-electrical-mechanical systems (MEMS) based accelerometers are also available. Tools may also include other sensors for determining properties such as wellbore temperature and azimuthal direction of inclination of the wellbore. For example, gyroscopes or magnetometers may also be included. The sensors are operated on power provided by a rechargeable battery pack located in the tool. Data recorded from the sensors is sent to onboard memory. The data is coded into pulses similar in theory to Morse code. Each MWD system has its own proprietary code.
A pulser located in the MWD tool is provided to generate hydraulic pulses at the MWD tool. The pulses create waves in the fluid mud column that reach the surface, where pressure sensors record them. The recorded pulses are filtered to remove normally occurring extraneous signal noise unrelated to the pulses sent by the MWD tool. The filtered pulses are then decoded to reveal the data recorded by the MWD sensors. The data is then displayed at the surface in a manner useful to the drilling personnel.
Pulses may be generated in a variety of manner, including positive pulse, negative pulse, and combinations thereof. Most conventional MWD units have a pilot valve that actuates a primary pulser. In this manner, a stronger pulse can be sent into the mud column which permits detection and identification apart from system noise and decoding by the surface equipment.
MWD tools are available in retrievable or non-retrievable designs. Retrievable MWD tools can be removed from the drill string at any time. Non-retrievable MWD tools remain in the drill string until the drill string is removed from the wellbore. Upon retrieval, MWD tools frequently require mechanical and electrical service.
Conventional pulser units include solenoid and motor driven varieties. Conventional motor driven pulsers utilize a magnetic coupling to transmit power between the motor and a ball screw assembly. The ball screw assembly actuates a pilot valve which in turn operates the pulser.
Conventional MWD designs have a combined lubrication and pressure compensation system. The system is intended to equalize the internal pressure of the lubrication and compensating reservoir with the external wellbore pressure, while using the compensating fluid for lubrication of the actuator mechanism. A pressure balance rod on which the pilot valve is attached is exposed to both internal and external pressures, and operates better when the internal and external pressures are balanced. Without a balanced system, the motor and actuator must overcome the pressure differential, with energy supplied by the limited batteries source. The reservoir also serves as the lubrication system for the actuator assembly, substantially enclosing the magnetic coupler and ball screw assemblies.
Due to the harsh environment in which survey tools operate, they are carefully sealed to protect the internal components. When motors are used, special seals are required to prevent the oil from leaking past the magnetic couplers into the motor. The lubrication and compensation reservoir is typically vacuum-filled and sealed on a shop bench.
A principal disadvantage of known MWD tools is the susceptibility of pilot valves clogging due to lost circulation material (LCM) becoming trapped between the pilot valve and the seat of the valve. In particular, MWD tools utilizing motors and magnetic couplings are limited in torque to the power of the magnet. Magnetic couplings are known to slip in high torque conditions, including interference caused by LCM, and thus rendering false pulse patterns. Magnetic coupling systems are also relatively long, commonly being up to three feet in length.
A principal disadvantage of the combined lubrication and pressure compensation system of known MWD tools is reliability and serviceability. When direct drive motors are used instead of magnetic couplings, the pressure compensating and lubrication oil must be sealed from the rotating shaft of the motor. The seal against the rotating shaft will have a limited life. The brushes and field coils of the motor will eventually pick-up the metal-iron fines and impurities in the oil, causing the motor to fail. When the motor fails, servicing the motor requires disassembly and drainage of the lubrication and compensation reservoir and, thus, return of the tool to the shop bench for sealing, reassembly and reservoir refilling under vacuum. Conventional MWD tools with lengthy components and integrated lubrication and compensation reservoirs are not, thus, serviceable in the field.
Another principal disadvantage of known MWD tools is the pressure loss associated with the location of the tool within the confines of the internal diameter of drill collars. Pressure is lost due to high flow rates between the exterior of the MWD tool and the drill collar I.D. This pressure is then unavailable to the drill bit. Pressure (or flow rate) to the drill bit is critical to the rate of penetration and life of the drill bit, and is a significant factor on the calculation of the cost per foot of the drilling operation. The larger the diameter of the MWD tool, the greater the system pressure loss will be. Similarly, the longer the MWD tool, the greater the system pressure loss will be.
Another principal disadvantage of known MWD tools is material cost. Due to the high velocity of the fluid between the exterior of the MWD tool and the inside diameter of the drill collars, expensive alloyed materials are required. Typically, the housing of conventional MWD tools is made from beryllium copper or a similarly wear resistant material. As a result, larger MWD tool diameters and longer length tools substantially increase the material cost of the tool.
A principal disadvantage of known MWD tools utilizing motors is that commercially available motors have a significantly larger form factor (profile) than do solenoid systems, thus requiring larger diameter housings and increased material cost. Also, higher-powered commercially available motors capable of extended service are larger in diameter. Additionally, motors are provided with mounting brackets that are external to the circumference of the motor housing. Therefore, increasing the motor power requires increasing the diameter of the tool. Of similar disadvantage, magnetic couplings are relatively lengthy assemblies.
Therefore, there is a further need to develop an MWD tool which can be serviced at the rig floor without draining the lubrication and compensation system, and without requiring shop delivery to reassemble and refill. There is a further need to develop an improved MWD tool having a shorter length to save cost and drilling efficiency. There is also a need to develop an MWD tool having greater reliability obtained from a more powerful motor driven actuator, without the increasing manufacturing and drilling costs associated with increased tool diameter. Lastly, there is a further need to develop a more compact and effective pressure compensation system.
There is also a need to accomplish these goals at a reasonable cost. The harsh drilling environment has prevented efforts to accomplish these goals in the past.